The invention relates to a spring-biased, automatic belt tensioner with a pivot-arm but more particularly, the invention relates to a tensioner having an arcuate bushing that provides bearing support and damping to pivoted movements of the pivot-arm.
Automatic belt tensioners for automotive use have a pivot-arm that is pivotally mounted at a base. A pulley is rotatably mounted to the pivot-arm and a spring between the pivot-arm and base biases the position of the pivot-arm to place the pulley against an endless belt of a belt drive system to tension the belt. It is preferred in some applications to have a bushing for a pivot of the pivot-arm in a common radial plane with the pulley to simplify bearing loads. Tensioners with pivot bushings and pulleys aligned in a common radial plane have evolved with pivot-arm lengths that may be classified into three distinct categories. In a first category, the length of the pivot arm is less than the radius of the inside diameter of a bearing that rotatably mounts the pulley. Such tensioners are sometimes referred to as eccentric type tensioners. In a second category of tensioners, the length of the pivot-arm is less than the radius of the pulley and are sometimes referred to as "pivot-in-pulley" or "short-arm" tensioners. A third category of tensioners have a pivot-arm length that is greater than the radius of the pulley.
The size of such belt tensioners largely depends on the length of the tensioner's pivot arm. The shorter the pivot-arm, the smaller the tensioner, and conversely the longer the pivot-arm, the larger the tensioner. If the pivot-arm is too short, the tensioner will not provide the necessary "take-up adjustment" to accommodate belt manufacturing tolerances, belt installation tolerances, and belt stretch during belt operation. If the pivot-arm is too long, the tensioner may not be able to fit within an allotted space for the tensioner.
The smallest size tensioners are the eccentric type of the first category where a pivot and effective pivot-arm are located within the confines defined by the inside diameter or bore of a pulley bearing of the tensioner. An example of an eccentric type belt tensioner with a minimum size bearing and pivot arm length is disclosed in U.S. Pat. No. 5,256,112. The length of the pivot-arm in tensioners in the first category may be increased by using a larger sized bearing which result in the bearing being oversized for the load that it must carry. An oversize bearing may result in increasing the necessary size for the tensioner as well as introducing excessive costs. Tensioners of the eccentric type may have pivot-arm lengths from about 5 millimeters to about 15 millimeters.
In a second category of tensioners, the axis for the pivot of the pivot-arm is located in a space generally limited by the outside diameter of the tensioner's pulley. Examples of "short-arm" tensioners in this second category are disclosed in U.S. Pat. No. 4,551,120 and U.S. Pat. No. Re. 34,543 (formally U.S. Pat. No. 4,824,421). A problem associated with these types of "short-arm" tensioners is that the pulley and pulley bearing usually need to be resized should it be necessary to change (increase or decrease) pivot-arm length. The tensioners of the '120 type generally have a pivot-arm length of about 30 to 35 millimeters. The tensioners of the '543 type allow a pivot-arm length of about 5 to 15 mm but the longer pivot-arm results in a large pulley diameter.
Within the third category of tensioners there is a type that has an arcuate bushing that provides bearing support and damping to pivoted movements of a pivot-arm. Such tensioners have an arcuate bushing at a large radius to provide a necessary damping to pivot-arm movements, and in some designs, a torsional spring surrounds the arcuate bushing at a even larger radius. The length of the pivot-arm in such tensioners must be sufficient to clear the radius of the arcuate bushing and the larger diameter torsional spring. Such tensioners typically have a pivot-arm length of about 75 mm. to about 100 mm. and are sometimes referred to as "long arm" tensioners. A "long arm" tensioner is disclosed in U.S. Pat. No. 4,723,934 (and B 1 4,723,934). While "long-arm" tensioners have proven satisfactory in several applications and have desirable economic advantages because they lack complex damping mechanisms, there inherently long pivot-arm (75 mm. to 100 mm.) somewhat limits their use because the tensioner is too large to fit within an allotted space of many automotive applications. In other words, a problem associated with such tensioners is that they are too big because they require a pivot-arm that is too long.
The present invention relates to tensioners in the third category and of the type with arcuate bushings that provide bearing support and damping to pivotal movements of a pivot-arm. The present invention solves the size problem associated with such "long-arm" tensioners by a design that permits the option of shortening the pivot-arm length to be nearly equivalent that of a "short-arm" tensioner in the second category while retaining requisite damping of pivoted movements of the pivot-arm along with an arcuate bushing.